Production of nanostructured materials

ABSTRACT

The invention relates to a method for the production of materials. In particular the invention relates to nanostructured materials, and an apparatus and method for the production thereof. In accordance with the invention, nanostructured materials are produced by the subsequent steps of producing nanoparticles; transporting the nanoparticles into, and optionally through, a porous carrier by a gas flow; and depositing the nanoparticles onto the surface of said porous carrier in an essentially isotropic manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 15/527,902filed on May 18, 2017, which is a 371 of PCT/NL2015/050814, filed Nov.20, 2015, which claims the benefit of priority from Netherlands PatentApplication Serial No. 2013836, filed Nov. 20, 2014, the contents ofeach of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for the production of materials. Inparticular the invention relates to nanostructured materials, and anapparatus and method for the production thereof.

BACKGROUND OF THE INVENTION

Nanostructured materials find important applications in society. Theymay for instance be present in catalysts, filters, sensing devices andelectronics. In general, it is important that the structures of thesematerials are well defined. This requires methods and apparatuses thatenable the production of the nanostructured materials in awell-controlled manner, preferably down to the nanoscale.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a versatile methodfor the production of nanostructured materials by depositingnanoparticles onto a porous carrier in an essentially isotropic manner.An essentially isotropic manner in the context of the present inventionis a manner that results in deposition of particles on the surfaces ofthe carrier facing the gas flow (“external side”) as well as on thesurfaces not facing the flow (“internal side”). Hence, on the scale ofthe pore size of the porous carrier, the nanoparticles are evenlydistributed throughout the porous carrier on both external and internalsurfaces. The internal surface is the part of the total surface of theporous carrier that is not in the “line-of-sight” from the source or theside from where the nanoparticles are produced. Of course, betweensource and deposition the duct may be curved, so line-of-sight is not tobe understood in the optical sense.

Methods known to date for the production of nanoscaled materialstypically involve the controlled growing of matter onto carriers. Atypical example of such technologies is physical vapor deposition (PVD).However, these technologies are generally restricted to the controlledgrowing of matter, e.g. nanoparticles, onto the external surfaces ofcarriers and as such limited to non-porous-carriers, e.g. flat surfacecarriers. In the case porous carriers are used, the deposition of thematter does not occur in an essentially isotropic manner as thepenetration of the matter into the porous carrier is limited.

For instance, US-A-2013/0045155 discloses the deposition of goldnanoparticles onto nanoporous materials by using the PVD method. It isdescribed that penetration of the gold nanoparticles is limited to about90 nm in depth and the major part of the carrier's surface is notcovered by the gold nanoparticles, so an anisotropic deposition isobtained. Hence, no essentially isotropic deposition is achieved.

US-A-2005/255242 describes the generation and deposition of atomic ormolecular vapor onto a substrate. A smooth layer of deposited vapor andno nanostructured material is obtained.

Alternative methods for the deposition of matter onto porous carriersinclude impregnation with nanoparticle-comp rising liquids. However,these methods do not result in the required control for the productionof nanostructured materials with a desired morphology.

The present inventors have surprisingly found a method that enables theproduction of nanostructured materials comprising nanoparticles that areessentially isotropically deposited onto a porous carrier. Thus, thepresent invention is directed at a method for producing nanostructuredmaterials comprising the step of:

a) producing nanoparticles;

b) transporting the nanoparticles into, and optionally through, a porouscarrier by a gas flow;

c) depositing the nanoparticles onto the surface of said porous carrierin an essentially isotropic manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the pathway of depositednanoparticles in accordance with the present invention.

FIG. 2 is a schematic representation of an apparatus which may be usedin a particular embodiment of the present invention.

FIG. 3 shows a nanostructured material in accordance with a specificembodiment of the present invention.

FIG. 4 shows another nanostructured material in accordance with aspecific embodiment of the present invention.

FIG. 5 shows yet another nanostructured material in accordance with aspecific embodiment of the present invention.

FIG. 6 shows the nanostructured material also shown in FIG. 5 at asmaller scale.

FIG. 7 shows yet another nanostructured material in accordance with aspecific embodiment of the present invention.

FIG. 8 shows the nanostructured material also shown in FIG. 7 at asmaller scale.

DETAILED DESCRIPTION OF THE INVENTION

The inventors found that the production of the nanoparticles can be verywell controlled. This means that a desired concentration ofnanoparticles with a desired size and shape can be produced. This isbeneficial to obtain the essentially isotropic distribution of thenanoparticles in the porous carrier.

With essentially isotropic is meant herein that, on the scale of thepore size of the porous carrier, the nanoparticles are evenlydistributed on the external and internal surfaces of the porous carrier.The term essentially indicates that the amount of depositednanoparticles may be different at the internal surface compared to theexternal surface but that, on the scale of the pore size of the porouscarrier, the thickness of the layers of deposited nanoparticles on theinternal and external surfaces are the same.

The even distribution of the nanoparticles on the scale of the pore sizemeans that a varying distribution at larger scale (e.g. centimeterscale) may be possible. For instance, due to the decrease of availablenanoparticles that can be deposited on surface throughout the porouscarrier, the amount of nanoparticles deposited deeper into the porouscarrier—when viewed from the flow of the nanoparticles—may be less thanat the beginning of the porous carrier. As such, the amount of depositednanoparticles deeper into the carrier may be less than at the beginningof the carrier, and at this large scale an even distribution is thus notrequired.

The scale of the pore size of the porous carrier depends on the porouscarrier that is selected for the present invention. Typical pore sizesare in the range of 1 to 1000 μm.

Nanostructured herein means a structure wherein individual particles of1-100 nm, preferably 3-30 nm, can be distinguished. As such, on thescale of the pore size of the porous carrier (or smaller), a roughsurface can be observed that is typically structured like a fractal.This fractal-like structure on this scale is in strong contrast to thesmooth surfaces that are obtained by e.g. physical vapor deposition.

In accordance with the present invention preferably a spark ablationdevice is used for the production of the nanoparticles and thenanostructured materials according to the present invention.

Spark ablation is a physical phenomenon whereby a spark between twoelectrodes is generated, leading to local ablation of the electrodes. Bythis ablation, a vapor of the matter comprised by the electrodes isobtained, which is subsequently rapidly cooled. This vapor initiallycomprises mainly single atoms, which collide and grow in time to largerparticles comprising multiple atoms, in a process described ascoagulation. The size to which particles grow by this mechanism dependsmostly on the initial vapor concentration and the time allowed forgrowth. By approximation, the mean particle mass m at a certain time canbe calculated by dividing the initial vapor mass concentration C by theparticle number concentration N at time t. From coagulation theory,which is assumed to apply to the present invention, it follows that theparticle number concentration essentially decreases proportional to 1/t,so that m=C|N, which is proportional to C×t. By working with shortprocess times, small particle masses, and thus small particle sizes, canbe obtained, while keeping the concentration high. High particleconcentrations result in larger production rates, and a faster, moreefficient deposition process.

The inventors found that in order to deposit the nanoparticles in anessentially isotropic manner, the particles should be highly diffusive.For particles smaller than the mean free path, the diffusion coefficientis proportional to the inverse of the square of the particle size, i.e.smaller particles are significantly more diffusive. For an essentiallyisotropic distribution, the size of the nanoparticle is preferably lessthan the mean free path of a molecule of the gas flow, more preferablythe size of the nanoparticle is less than 20 nm, most preferably lessthan 10 nm. The mean free path is the mean length of a path covered by agas molecule of the carrier gas between subsequent impacts with othermolecules. The mean free path is dependent on the mass of the molecules,the pressure and temperature. For instance, the mean free path of an airmolecule at ambient temperature and pressure is 68 nm.

It is preferred for the present invention that the nanoparticles aregenerated and deposited at a pressure of 1 to 2 atmosphere, morepreferably at atmospheric pressure. As such, no vacuum installation isrequired allowing a more convenient method for the preparation of thenanostructured materials.

Hence, following the coagulation theory described above, nanoparticlescan be prevented from growing too large by using low concentrations ofdepositing nanoparticles. However, the rate of deposition of thenanoparticles onto the porous carrier is proportional to theconcentration of the depositing nanoparticles. Hence, a lowconcentration of nanoparticles results in slow deposition thereof ontothe porous carrier and thus in time consuming methods. Therefore, it ispreferred to avoid low concentrations of the nanoparticles.

If the process time is kept short enough, high particle concentrationscan be combined with particle sizes sufficiently small for essentiallyisotropic deposition. Hence, the time between when the initial cloud ofmaterial or vapor is formed and when the particles are deposited ispreferably minimized. In a preferred embodiment of the presentinvention, the nanoparticles are deposited in a time of less than 1000ms, preferably less than 100 ms, more preferably less than 75 ms,typically 10-50 ms after the production of the nanoparticles. Such shorttimes allow high concentration of the nanoparticles, while sufficientlylimiting particle growth to ensure efficient deposition.

It was found that by using a spark ablation device the size andconcentration of the depositing particles can be controlled whilemaintaining such short deposition times. Because the volume of theinitial vapor cloud may be small, the particles can be effectivelytransported to the deposition area. The transport time can be short, sothat the nanoparticles grow no larger than the desired size.Additionally, the residence time distribution is narrow, i.e. eachdepositing particle has a similar growth history, resulting in excellentcontrol of particle size. A spark ablation device which may beparticularly suitable for the present invention is described inWO-A-2013/115644, which is incorporated herein in its entirety.

The size of nanoparticles may be controlled by the specific use of thespark ablation device and the gas flow. In particular, the desired sizeof the nanoparticle may be set on the one hand by varying spark energy(J), and spark-frequency (sparks/s), determining the production rate,and on the other hand by gas flow rate (dm³/s), or by a combination ofthese parameters (Pfeiffer et al. “New Developments in Spark Productionof Nanoparticles.” Advanced Powder Technology 25(2014) 56-70). The sparkenergy (typically 0.1 mJ-1 J), spark-frequency (typically 50-50.000spark/s) and gas flow rate (typically 0.01-10 dm³/s) may also beparameters used to control the concentration of nanoparticles. Theseparameters may be used independently to control the concentration ofnanoparticles. Typically, the concentration of nanoparticles isprecisely controlled by changing both the spark energy and the sparkfrequency or by changing all three parameters.

Preferably, the gas flow is chosen small enough to avoid damage to theporous carrier. Using low pressure drop porous substrates limits theforce exerted by the flowing gas on the porous carrier, allowing the useof greater gas flows and thus shorter residence times. Preferably, themean face velocity through the porous medium is about 0.1 to 5 m/s.

The use of a spark ablation device is particularly advantageous overmethods for the production of nanoparticles that rely on heating the gasstream. The rapid cooling in the spark enables fast deposition of theparticles on the porous structure at low temperature, typically roomtemperature, which is much lower than the temperature at which the vaporis formed. The fast deposition achievable with the present invention (ashort residence time) limits particle growth by coagulation, allowingnanoparticles to be deposited at high production rates and at feasiblegas flow rates. Most alternative methods for the production ofnanoparticles cause substantial heating of the gas stream, so that afast deposition results in melting of deposited nanoparticles which inturn may result in a smooth layer of the deposited nanoparticles. Asmooth layer may be unfavorable in applications of the nanostructuredmaterial since the surface area of the layer is lower when it is smooth.For instance, if the application is catalysis, a large surface area isusually advantageous for catalytic activity. If the application issensing, a large surface area may be advantageous for a high sensitivityof the sensor. High temperatures may also damage the porous carrier.Hence, a low temperature downstream of the production of thenanoparticles is advantageous to obtain an open structure of thenanostructured materials. Therefore, in a preferred embodiment of thepresent invention, the temperature of the gas flow at the point ofdeposition is lower than 200° C., preferably lower than 100° C., mostpreferably lower than 50° C., even more preferably between 10 and 30°C., typically about room temperature (25° C.).

The materials of which the nanoparticle may be composed, may becontrolled by selecting electrodes comprising the correspondingmaterials. Hence, for producing a nanoparticle comprising gold, anelectrode comprising gold may be selected for the spark ablation device.A spark ablation device requires two or more electrodes. By selectingelectrodes comprising different materials, nanoparticles may thus beproduced comprising a variety of materials. For instance, ifnanoparticles comprising gold and nanoparticles comprising aluminumwould be required, an electrode comprising gold and an electrodecomprising aluminum could be selected for use in the spark ablationdevice. Hence, in a preferred embodiment of the present invention, thematerial comprised by the nanoparticles, may be selected by selectingappropriate electrodes for the spark ablation device. This enablesprecise control on the material comprised by the nanostructuredmaterials produced by the present invention. Hence, nanostructuredmaterials comprising a layer of nanoparticles that comprises two or morematerials may also be obtained by the present invention. One materialmay form the carrier material of a catalyst and the other material maybe the active catalyst.

Chemically modified nanoparticles may be obtained by reacting theparticles with a gas, e.g. oxygen, before or after deposition. Coatednanoparticles may be obtained by adding nanoparticles to the gas flowentering the inlet into the reactor.

For a typical spark ablation device comprising two electrodes, theamount of nanoparticles produced from each electrode are not identical.Typically, the amount of nanoparticles are produced in a 3:1 ratio fromeach electrode. This may depend on the relative position of theelectrodes to the direction of the electrical current, as well as onmaterial properties of the respective electrodes. Hence, the relativeposition of the electrodes may also be of influence for the compositionof the layer of deposited nanoparticles. This may be of particularinfluence in case the electrodes each comprise different matter.

In another embodiment of the present invention, the used electrode maybe switched or substituted by another electrode during the process. Thisenables additional control of the structure of the nanostructuredmaterials. By switching to an electrode comprising a different material,the nanostructured material produced may comprise layers of differentmaterials. The thickness of each layer (i.e. the amount of depositednanoparticles) may be controlled by the duration of the production ofthe nanoparticles with the corresponding electrode.

WO-A-2013/115644 also describes how big droplets of materials may beavoided by using magnetic fields. This may also be advantageous for thepresent invention. The use of one or more hollow electrodes may also bepreferred. Hollow electrodes may reduce the residence time of thenanoparticles in the spark ablation device. As a consequence, the timebetween the production of the nanoparticles and the deposition of theseonto the porous carrier may be reduced by using a spark ablation devicecomprising hollow electrodes. In particular, said time may be reduced bydirecting the gas flow through the hollow electrode or hollowelectrodes.

The nanoparticles are deposited onto a porous carrier. The porouscarrier may comprise any type of material. Typical materials may bemetal, ceramics or organic polymers such as polyvinyl alcohol or carbonnanotubes. Combinations of different materials may also be possible.

Preferably, the porous carrier is a nanoporous carrier, preferablycomprising nanofibers or nanowires.

In the particular embodiment that the porous carrier comprisesnanowires, the thickness of the nanowires is preferably less than 1000nm, more preferably less than 250 nm. Typically, the thickness of thenanowires is in the order of or less than about the mean free path of amolecule of the flow gas. In a preferred embodiment, the thickness ofthe nanowire is between 70 and 200 nm. Nanowires of these particulardimensions are particularly advantageous for a low pressure drop whenthe nanoparticles are transported into the porous carrier by the gasflow. For fibers of this thickness the efficiency of deposition by meansof diffusion for particles smaller than about 20 nm is 5-1000 timesgreater than unity, i.e. the fibers collect nanoparticles from an areagreatly exceeding the projected area of said fiber. If the nanowires aretoo thick, said transportation of the nanoparticles would be inhibitedand thereby the deposition of the nanoparticles may not be in anessentially isotropic manner. For this reason, the void between saidnanowires may also be sufficiently large in order to avoid a largepressure drop of the gas flow. With thinner wires a lower volumefraction of carrier material can be achieved, resulting in very highrelative concentrations of deposited nanoparticles in the nanostructuredmaterial.

The porous carrier according to the present invention may be prepared byelectrospinning polymers onto a clean mesh structure that addsmechanical strength.

The inventors found that by small dimensions of the nanoparticles andthe porous carrier, the deposition of the nanoparticles is particularlyessentially isotropic. This may be effected by diffusion of thenanoparticles. Therefore, in a particular embodiment of the presentinvention, the nanoparticles are deposited onto the porous carrier in anessentially isotropic manner by diffusion of the nanoparticles. In thecontext of the present invention, diffusion of the nanoparticles meansthat the nanoparticles are partially passively dispersed throughout theporous carrier. Hence, although the nanoparticles are activelytransported into the porous carrier by the gas flow, advantageously thenanoparticles also move by means of diffusion to deposit onto the porouscarrier. If the nanoparticles are sufficiently small, diffusion isdominant with respect to other transport mechanisms, in particular thosebased on particle inertia. Without wishing to be bound by theory, theinventors believe that this may be caused by the Brownian motion of thenanoparticles.

FIG. 1 depicts the cross-section of a single fiber (1) suspendedperpendicularly to a flow of gas. Shown are the initial positions (2) ofa large and a small particle on their respective streamlines (3). Thediffusivity of the large particle is negligible, and it follows thestreamlines until it is deposited on the external surface of the fiberby interception (4), often aided by inertial forces. The small particlehas a significant diffusivity, and after a random walk (5) is depositedisotropically onto the fiber (6). Although a single fiber is depicted,the same processes are applicable to a structured or random network ofmultiple fibers. It will be appreciated that the isotropic depositionalso applies to surfaces placed downstream, i.e. in the shadow ofexternal fibers. A preferred configuration of an apparatus for theessentially isotropic deposition of nanoparticles is provided in FIG. 2.It may comprise an ablation chamber (7) equipped with one or two gasinlets (8,9), and one or more pairs of axially aligned electrodes (10 a,10 b). Each of the electrodes (10 a, 10 b) may comprise at least oneconducting or semiconducting material. Electrode 10 a is optionally asolid rod, and optionally of the same composition as electrode 10 b. Oneor both electrodes may be placed at high potential. The vapor cloudablated by each spark may be transported from the ablation zone (11)through a nozzle (12) into the deposition chamber (13 a,13 b).Preferably, the walls of the chambers (7, 13 a, 13 b) and the nozzle aremade of a conductive material, and are typically kept at groundpotential. The porous carrier (14) may be supported such that the gasflow is forced to flow through the carrier from section (13 a) to (13b). After passing through the porous carrier (14), the gas may exitthrough exhaust port (15). The residence time of the gas can becalculated by dividing the volume (dm³) separating the ablation zone(11) and the porous carrier (14), i.e. the sum of the inner volumes ofsections 10 b, 12 and 13 a, by the volumetric flow rate (dm³/s).

If appropriate material for the porous carrier is selected, the porouscarrier may be removed after deposition of the nanoparticles. Forinstance, in case the porous carrier is composed of organic materials,it may be oxidized such that only the layer of deposited nanoparticlesremains.

A further aspect of the present invention is directed to nanostructuredmaterials. In particular to nanostructured material comprising a porouscarrier and an essentially isotropic layer of deposited nanoparticles onsaid porous carrier.

A particular embodiment of the present invention is a nanostructuredmaterial comprising a porous carrier and an essentially isotropic layerof deposited nanoparticles on said porous carrier, wherein the layer ofnanoparticles has a thickness between 0.01 μm and 10 μm, preferablybetween 0.1 μm and 7 μm, more preferably between 0.7 μm and 5 μm, mostpreferably between 1 μm and 3 μm. Typically, the deposited layer has atotal thickness significantly thicker than the smallest dimension of theporous carrier (e.g. thickness of the wire). Due to the rough surface orfractal-like structure of the nanostructured material, the thickness ofthe deposited layer can vary significantly on a nanoscale, see forinstance the figures. The thickness of the deposited layer is thereforeideally determined at a micrometer scale.

The nanostructured material according to the present invention may beused in catalysis, sensing, detecting, filters, electrical devices andthe like.

A poster entitled “A new, clean and flexible method of catalystsynthesis” which was presented by two of the present inventors at TheNetherlands' Catalysis and Chemistry Conference XV at 10 Mar. 2014,disclosed that deposition of spark produced nanoparticles on top ofelectrospun nanofibers is highly efficient. Transportation into a porouscarrier and depositing the nanoparticles throughout the porous carrierin an essentially isotropic manner according to the present inventionhas not been disclosed.

For the purpose of clarity and a concise description, features aredescribed herein as part of the same or separate embodiments. However,it will be appreciated that the scope of the invention may includeembodiments having combinations of all or some of the featuresdescribed.

The invention is further illustrated by the following experimentalexamples.

EXAMPLE 1

A spark was generated between two gold electrodes at a repetitionfrequency of 900 Hz. The mean power consumed by the spark was 1 W. Using4 dm³/min of argon flow and a residence time of 47 ms between the sparkand the porous carrier, nanoparticles were deposited during 4 hours on a14 mm porous carrier consisting of polyvinyl alcohol (PVA) nanofiberswith diameters of 50-200 nm supported on a steel mesh. An essentiallyisotropic nanoporous structure with a layer thickness of 2 μm wasobtained (FIG. 3).

EXAMPLE 2

A spark was generated between one gold and one aluminium electrode at arepetition frequency of 300 Hz. The mean power consumed by the spark was1.5 W. Using 3 dm³/min of argon and a residence time of 50 ms,nanoparticles were deposited during 2 hours on a 14 mm porous carrierconsisting of PVA nanofibers with diameters of 50-200 nm supported on asteel mesh. Upon exposure to air, an essentially isotropic nanoporousstructure comprising a mixture of Al₂O₃ and Au nanoparticles with alayer thickness of 1 μm was obtained (FIG. 4).

EXAMPLE 3

Sparks were generated between two aluminum electrodes at a repetitionfrequency of 1000 Hz. The mean power consumed by the spark was 3.5 W.Using a gas flow of 7 dm³/min comprising 95% Ar and 5% O₂ and aresidence time of 900 m s, nanoparticles were deposited during 25minutes on a 14 mm porous carrier consisting of PVA nanofibers withdiameters of 200-400 nm supported on a steel mesh. An essentiallyisotropic nanoporous structure of Al₂O₃ nanoparticles with a layerthickness of 0.5 μm was obtained (FIGS. 5 and 6).

EXAMPLE 4

Sparks were generated between one titanium electrode and one goldelectrode at a repetition frequency of 830 Hz. The mean power consumedby the spark was 3.75 W. Using a gas flow of 7 dm³/min comprising 95% Arand 5% O₂ and a residence time of 900 m s, nanoparticles were depositedduring 25 minutes on a 14 mm porous carrier consisting of PVA nanofiberswith diameters of 200-400 nm supported on a steel mesh. An essentiallyisotropic nanoporous structure of 5-10 nm titanium dioxide nanoparticlesand 2-5 nm gold nanoparticles with a layer thickness of about 0.5 μm wasobtained (FIGS. 7 and 8).

1. A nanostructured material comprising a porous carrier and asubstantially isotropic layer of deposited nanoparticles on said porouscarrier.
 2. The nanostructured material of claim 1, wherein thenanoparticles are evenly distributed on the external and internalsurfaces of the porous carrier.
 3. The nanostructured material of claim1, wherein wherein the nanoparticles range in size from 1 to 100 μm,thereby providing the nanostructured materials with a rough surface. 4.The nanostructured material of claim 1, wherein the layer ofnanoparticles has a thickness of between 0.01 μm and 10 μm.
 5. Thenanostructured material of claim 4, wherein the layer of nanoparticleshas a thickness of between 0.1 μm and 7 μm.
 6. The nanostructuredmaterial of claim 5, wherein the layer of nanoparticles has a thicknessof between 0.7 μm and 5 μm.
 7. The nanostructured material of claim 6,wherein the layer of nanoparticles has a thickness of between 1 μm and 3μm.
 8. The nanostructured material of claim 1, wherein the layer ofnanoparticles comprises two or more materials.
 9. The nanostructuredmaterial of claim 1, wherein the nanoparticles are produced with a sparkablation device.
 10. The nanostructured material of claim 1, wherein thenanoparticles are deposited onto the porous carrier by diffusion of thenanoparticles.
 11. The nanostructured material of claim 1, wherein theporous carrier is a nanoporous carrier.
 12. The nanostructured materialof claim 1, wherein the size of the nanoparticles is less than the meanfree path.
 13. The nanostructured material of claim 11, wherein thenanoporous carrier comprises nanowires.
 14. The nanostructured materialof claim 13, wherein the nanowires have a diameter of less than 1000 μm.15. The nanostructured material of claim 14, wherein the nanowires havea diameter of less than 200 μm.
 16. The nanostructured material of claim13, wherein the nanowires have a diameter of less than the mean freepath.
 17. The nanostructured material of claim 1, wherein the size ofthe nanoparticles is less than 20 μm.
 18. The nanostructured material ofclaim 17, wherein the size of the nanoparticles is less than 10 μm. 19.The nanostructured material of claim 1, wherein the nanostructuredmaterial comprises two or more types of nanoparticles formed fromdifferent materials.